C Tutorial

C Tutorial C Language Environment Setup Execution flow of C program C printf and Scanf C Data type C Token Variable in C Operators in C Comments in C Escape Sequence in C C – Storage Classes C Decision control statement Loop Statement in C Break, continue and goto statement in C Type Casting in C Function in C Recursion in C String in C C Array Pointer in C Dynamic memory allocation C –Structure Nested Structure in C Union in C File Handling in C C pre-processor Static Function In C Sizeof In C Selection Sort In C Scope Of Variables In C Runtime Vs Compile Time In C Random Access Lseek In C Queue Implementation In C Pseudo Code In C Prototype In C Pointer To Pointer In C Pointer Arithmetic In C Passing Array To Function In C Null Character In C Merge Sort In C Macros In C Library Functions In C Memory Leak In C Int In C Goto And Labels In C Fibonacci Series In C Fflush In C Derived Data Types In C Data Types In C Const Vs Volatile In C Character Set In C Character Class Tests In C Calloc In C C Pointers Arrays In C Include In C Clrscr In C C Vs Java String Literals In C Types Of Pointers In C Variables In C Volatile In C Why C Is A Middle Level Language Infix To Postfix Program In C Ceil function in C LCM of two numbers in C Quick sort in C Static in C function pointer as argument in C Top Array Keywords in C Add two numbers using the function in C Armstrong program in C using function Array, Declaring Arrays and Array Initialization Limitations of Inline Function in C Merge and Merge sort with example in C Do-While Loop in C For Loop in C While-Loop in C Difference between while and do-while loop in C Array Of Structures in C Data Structures And Algorithms in C Types Of Structures In C How to Avoid Structure Padding in C Use of Structure in C Do WHILE LOOP in C Programming Examples For Loop in C Programming Examples Entry Control Loop in C Exit control loop in C Infinite loop in C Nested loop in C pow() function in C String Handling functions in C Prime Number code in C Factorial Program in C using For Loop Factorial Program in C Using While Loop Fibonacci Series in C Using For Loop Fibonacci series in C using while loop Prime Number Program in C using for Loop While Loop in C programming examples Built-in functions in C Assert() Function C vs Java Strings Call Back Function in Embedded C Else If Ladder fgets() function Ftell() Function getc() function getch() function gets() function Heap Sort Nested if-else statement Pi() Function Positioning of file Write() function abs() function in C Attributes in C C program to find factorial of a number using Recursion Ferror() in c fopen() function in C Fibonacci series program in C using Recursion Formatted Input and output function in C Snake Game in C User Defined Functions in C Beep() function in C Cbrt() function in C Hook() function in C Isalnum() function in C C Program to find the Roots of a Quadratic Equation C Switch Statements Difference between rand() and srand() function in C Difference between while and for loop in C Doubly Linked list in C Example of Iteration in C How to use atoi() function in C How to use floor() function in C How to use sine() function in C How to use Typedef Struct in C Integer Promotions in C C Program Swap Numbers in cyclic order Using Call by Reference C Program to Find Largest Number Using Dynamic Memory Allocation C Program to Find the Largest Number using Ternary Operator C/C++ Program to Find the Size of int, float, double and char Find the Largest Three Distinct Elements in an Array using C/C++ Loop Questions in C Modulus on Negative Numbers in C Multiplication table program in C using For loop Nested Loops in C Programming Examples C Program for Mean and Median of an Unsorted Array Results of Comparison Operations in C and C++ Reverse a Stack using Recursion in C Simple hash() function in C strcat() Function in C Sum of N numbers in C using For loop Use of free() function in C Write a program that produces different results in C and C++ C Function Argument and Return Values Keywords in C Bank management system in C Calendar application in C Floor() Function in C Free() Function in C How to delete a file in C How to move a text in C Remove an element from an array in C Unformatted input() and output() function in C What are linker and loader in C SJF Scheduling Program in C Socket Programming in C Structure in C Tower of Hanoi in C Union Program in C Variable Declaration in C What is Linked List in C While Loop Syntax in C fork() in C GCD program in C Branching Statements in C Comma Operator in C Control statement in C Double Specifier in C How to create a binary file in C Long int in C Palindrome Number in C Pure Virtual Function in C Run Time Polymorphism in C Types of Array in C Types of Function in C What is a buffer in C What is required in each C Program Associativity of Operators in C Bit Stuffing Program in C Actual and Formal Parameters Addition of two Numbers in C Advantages of function in C Arithmetic Progression Program in C Binomial Coefficient Program in C Difference between Array and List in C Diffie-Hellman Algorithm in C How to convert a number to words in C How to convert a string to hexadecimal in C Difference between If and Switch Statement in C C and C++ Binary Files C program that does not Suspend when Ctrl+Z is Pressed Different ways to Declare the Variable as Constant in C Range of Int in C C Program to find the size of a File FIFO Example in the C Language For loop in C Programming GCD program of two numbers in C GPA Calculator in C How to Calculate Time Complexity in C How to include graphics.h in C How to measure time taken by a function in C How to return a Pointer from a Function in C What is the main in C Addition of Matrix in C Booleans in C C Program for Extended Euclidean algorithms C Program of Fencing the Ground Ceil and Floor in C Compound Interest Program in C Displaying Array in C Distance Vector Routing Protocol Program in c Dos.h Header File in C Language DSA Program in C Explain the two-way selection in C Fee Management System in C File Operations in C Malloc function in C Multiplication Table in C Simple Programs in C Language tolower() Function in C Type Conversion in the C Why does sizeof(x++) not Increment x in C Advantages of Dynamic Memory Allocation in C Armstrong Number in C Assignment Operator Program in C Banker’s Algorithm in C Binary Search in C with Best and Worst Time Complexity Caesar Cipher Program in C Call by Value and Call by Reference in C Conditional Operator in C CRC Program in C Deadlock Detection Program in C Decimal to Binary in C Difference between If Else and Nested If Else in C Difference between Pre-increment and Post-increment in C Difference between Scope and Lifetime in C Evaluation of Arithmetic Expression in C Explain the Increment and Decrement Operators in C Fseek Function in C Functions in C How to Find Square Free Numbers in C Length of an Array Function in C OpenGL in C Projects on C language in 2023 Purpose of a Function Prototype in C Stdio.h in C Two-Dimensional array in C What is String Comparison in C C Compilers for Windows Functions and Recursion in C How to Declare Boolean in C How to Declare Character in C How to Round up a number in C How to use strlen() in C Pointer Declaration in C Algorithm for String Palindrome in C C Program to find ASCII value of a character Constant Pointer in C How to find string length in C using strlen() function Implicit and Explicit in C Indirect Recursion in C Input and Output functions in C isupper() in C Jump Statement in C Lifetime of a Variable in C Linker Error in C Language Numeric Constant in C Size of Pointer in C Square Root in C Language Static and Dynamic Memory allocation String Declaration in C Strong Number in C Symmetric Matrix in C Types of C Tokens What is a Size of Pointer in C What is Increment and Decrement Operator in C 1 2 3 4 Series Program in C Advantages and Disadvantages of C Language C Program for Polynomial Addition C Program to Count the Number of Vowels in a String C Programming Errors and Solutions Compilation Errors in C Complex C Programs Difference between Argument and Parameter in C Difference between char s[] and char *s in C Evaluation of Postfix Expression Using Stack in C Find Leftmost and Rightmost Set Bit of a Number fprintf and fscanf in C Introduction to Dynamic Array in C Print Address in C Realloc function in C Ternary Operators in C Types of Tokens in C with Examples Difference between Static and Dynamic Memory Allocation in C Addition Program in C Array Definition in C Array of Pointers in C Arrow Operator in C Average of Two Numbers in C Binary to Decimal in C Binary to Octal in C BREAK STATEMENT in C C Programming Operators Questions C Programs Asked in Interview Calculator Program in C C Program to Read and Print an Employee's Detail Using Structure Bubble Sort Algorithm in C C Program to Find Area and Perimeter of Circle C Program to Check Whether a Given Number is Even or Odd C in Roman Numerals C Program to Make a Simple Calculator Using Switch Case Insertion Sort Program in C How to take input in string in C GCC Conflicting Types in C Function Definition in C Format Specifier for Hexadecimal in C Flowchart in C Float in C Fizzbuzz Implementation in C Conditional Statement in C Conio.h functions list in C Constants in C Dynamic Array in C Decision Making Statements in C Continue Statement in C Creation of Thread in C DFS Algorithm in C Difference between parameter and arguments in C Dijkstra's Algorithm in C Leap Year Program in C Jump Statements in C Modulus Operator in C Memory Allocation in C Simple Interest Program in C Reverse Array in C Recursive Function in C Queue in C Printing Pascal’s Triangle in C Preprocessor Directives in C Perror() in C Perfect Number in C Programming Language Parameter Passing Techniques in C Pascal Triangle in C Patterm Program in C

Random Access - Lseek in C


lseek is a system call that is used to alter the location of the read/write pointer of a file descriptor. The lseek system call basically moves the current read/write location to anywhere within the referenced file.

The Input and output are normally sequential in the concept of file descriptors. Each read and write operation in this concept takes place at a position in the file right after the previous one. The file can be read or written in any random order in case it is necessary or required.

The lseek plays a vital role here by providing a way to navigate around in a file without reading or writing any information or data.

System call

long lseek (int fd, long offset, int origin);

The above piece of code is a system call which sets the current position in the file whose descriptor is fd to offset, which is taken relative to the location specified by origin. Here, offset is the number of bytes to move the selector.

The concluding read or write will begin at that position. The origin can be 0(zero),1 (one), or 2(two). It is numbered so, in order to specify that offset is measured from the beginning (top), from the current position (where current pointer points), or from the end of the file respectively.


In order to be adjoined to a file, we need to seek to the end before writing the following. This can be achieved by using the following piece of code;

lseek (fd, 0L, 2);  // seek to the end before writing

Note: In order to get back to the beginning, the term (“rewind”) is used,

lseek (fd, 0L, 0);

Note: In the above piece of code,the ‘0L’ argument here can be written as (long) 0 or just as (zero) 0 if the declaration of lseek is properly done.

Use of lseek in C

The lseek is quite useful in C programming and implementation of file concepts like fopen or getc. With the lseek(random access), it is possible to treat files like arrays (more or like large arrays). This may help the programmer but it costs in terms of slower access.

For example:

The following function reads the nth byte of a file and copies it to another file using the lseek.

Program 1:

#include <stdio.h>    //header files 
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>

void func(char arr[], int n)

int f_write = open ("start.txt", O_RDONLY);

int f_read = open("end.txt", O_WRONLY); 

int count = 0;

while (read(f_write, arr, 1))
if (count < n)
lseek (f_write, n, SEEK_CUR);
write (f_read, arr, 1);
count = n;
count = (2*n);
lseek(f_write, count, SEEK_CUR);
write(f_read, arr, 1);

int main()
	char array[50];  // Driver code
	int n;    //declaring a variable ‘n’ of type ‘int’
	n = 5;  //value assigned to n

	func(array, n);  //function call
	return 0;

Example 2:

#include <sys/types.h>    //header files

int get(int fd, long pos, char *buf, int n)

    if(lseek(fd, pos, 0)>=0)
    return read(fd, buf, n); 
    return -1;  //return ‘-1’ in case of error

Code explanation:

In the above code, the function reads any number of bytes from any arbitrary (or random) place in a file.

It returns the number read. In case an error occurs, it returns ‘-1’.

Return value of lseek

The return value of lseek is a long that gives the new position in the file. In case an error occurs, it returns ‘-1’. The standard library function fseek is similar to lseek. Of Course, there are some similarities as well;

  • The first argument is a file *
  •  In case an error occurs, it returns ‘-1’.

fseek function in C

fseek() function is another similar library function to lseek in C programming which is used to move file pointer position to the given location.

SEEK_CUR – Seek_Current is used to move file pointer positions to a given location.

SEEK_END – Seek_End is used to move file pointer position to the end of file.

Drawback of lseek

Is Lseek thread safe? The original whence, exactly as passed to the lseek(2) system call.

Note: This function is not thread safe, because it shares a return buffer across all threads, and many other functions in this library


Upon successful termination;

The lseek() or random access returns the resulting offset location as measured in bytes from the top or beginning of the file. In case an error occurs, it returns ‘-1’.